karin_mt_2

Countertop Geology, Part Six. Let's talk about rocks some more!

karin_mt
5 years ago

This is round six of the Great Rocks Thread!

Please post your rock questions here. I've copied the basic info about quartzite and marble here because this is the most frequent question.

Quartzite and marble are hopelessly (deliberately?) mixed up in the decorative stone industry. My point, aside from just loving rocks, is to help folks learn how to tell the difference between the two so you are not at the mercy of a sales rep when a multi-thousand dollar purchase hangs in the balance.

Quartzite is much harder than marble and will not etch when exposed to acids. You can tell the difference between quartzite and marble by doing the scratch test and the etch test.

Scratch Test
Take a glass bottle or a glass tile with you when you go stone shopping. Find a rough, sharp edge of the stone. Drag the glass over the edge of the stone. Press pretty hard. Try to scratch the glass with the stone.

Quartzite will bite right into the glass and will leave a big scratch mark.
Any feldspar will do the same. (Granites are made mostly of feldspar)

Calcite and dolomite (that's what marble and limestone are made of) will not scratch. In fact you will be able to feel in your hand that the rock won't bite into the glass. It feels slippery, no matter how hard you press.

PS - don't press so hard that you risk breaking the glass in your hand. You shouldn't need to press that hard!

Etch Test
Etching is when the surface of a rock is dissolved from acids like lemon juice, vinegar, wine, etc. It is the primary bummer about using marble in a kitchen. Etching is most noticeable on polished rocks. Etching is not prevented by sealers, no matter what you hear from the sales rep!

Doing the etch test is simple: bring home a sample of the rock and put lemon juice or vinegar on it. Even after a few minutes the results are usually obvious. Etched areas look duller and are discolored compared to the rest of the slab.

Some people get conflicting results with these two tests, but normally anything in the marble family will not scratch glass and it will etch.

Quartzite and rocks in the granite family will scratch glass and will not etch.

Super White, the rock that started this whole discussion, turns out to have small amounts of quartz in it. Not enough to make the rock durable, but just enough to confound your test results because it can kinda sorta scratch glass. Real quartzite really gouges glass and even makes a grinding sound as it does so. But Super White can scratch glass somewhat, in some cases. So always do the etch test too because even with a little quartz in it, Super White will still etch.

For reference, here are links to the other rock threads, in which many types of rocks have been discussed.

Rocks 101: The Lowdown on Super White

Rocks 102: Marble, Quartzite and Other Rocks in the Kitchen

Rocks 103: Countertop Geology: Marble and quartzite and granite, oh my!

Rocks part 4, Marble, Granite, Quartzite

Rocks part 5, Marble, Quartzite, and other favorites

With that, let the rock conversations continue!
-Karin

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